A sagging ceiling, creaky floor, or leaning wall in a home, office building, or other structure may be the result of any number of unenviable circumstances, including but not limited to poor original architectural design, substandard original construction or substandard renovation, aging materials, severe weather, or a shifting foundation. In any event, walls of many homes, offices, and other structures are not exactly vertical. Historically, a “floating shelf” has been a type of shelf that substantially hides its support brackets within itself, to appear as though it is floating against a wall. Wikipedia, https://en.wikipedia.org/wild/Floating_shelf.
Hanging a floating shelf on a leaning wall in a way that the shelf is level notwithstanding the lean of the wall has been undesirably challenging. Relevelling such a shelf after a wall has shifted has been undesirably challenging as well.